One of the important components in a thin film transistor display panel is the array substrate, and an image is displayed by using the thin film transistor to drive pixels of liquid crystal or organic light-emitting diodes. The array substrate is provided with the thin film transistor, a pixel electrode, a gate line (also called as gate scanning line) and a data line (also called as data signal line) interlaced with each other, as well as a storage capacitor for storing pixel voltage.
In order to obtain an image with good quality, an uniform electric field for the liquid crystals or light-emitting diodes is necessary at the pixel region (i.e. a region where the pixel electrodes are located) by means of a planarization layer. Generally, a protective passivation layer and a planar organic material layer are formed on a source-drain metal layer (i.e. a drain pad that is in the same layer as the source and drain of the thin film transistor and that serves as an electrode of the storage capacitor), and the pixel electrode connects to the source-drain metal layer through a via hole penetrating the passivation layer and the organic planarization layer, thereby forming an electrical connection.
However, the organic planarization layer usually has a large thickness ranging from 1 μm to 5 μm, the via hole penetrating both the passivation layer and the organic planarization layer need to be formed by an overlay process (i.e. a process for forming the via hole need to be repeated several times at the same position). Since the via hole is rather deep (over 1 μm), photoresist residues are liable to occur on the surface of the source-drain metal layer, this would cause bad contact between the pixel electrode and the source-drain metal layer, which in turn would result in bright or dark spots on the display screen when performing displaying by a display device with the array substrate.